Nitrogen fertilizer leaching from cropped and irrigated sandy soil in Central Turkey

Pollution of ground water caused by excessive and uncontrolled use of nitro gen fertilizer is worrying. A recent example of such pollution has been obs erved in an agricultural basin in the province of Nevsehir, Turkey, where u p to 900 kg ha(-1) nitrogen fertilizer is used for growing potatoes in sand y soils under irrigation. Using nitrogen fertilizer in amounts that guarant ee large yields without polluting ground water is essential. We present res ults of field experiments and numerical simulations involving N-15-labelled nitrogen fertilizer leaching. In the field, we monitored the movement of w ater and the distributions of nitrogen species within the soil-water-plant continuum. The detailed dynamics of the nitrogen cycle within the system we re simulated. Simulations included calibration and validation of the nitrog en version of the LEACHM model (LEACHN, version 3) and long-term applicatio ns of the model. The model's predictions of nitrogen fluxes under long-term use of fertilizer and irrigation were analysed. Nearly half of the applied ammonium-N was converted to nitrate-N during the growing season. With incr easing additions of N the rate of plant uptake declined, while leaching inc reased significantly, and the fraction of nitrogen remaining in the soil pr ofile increased only moderately. In long-term applications, a significant f raction of the applied fertilizer tended to accumulate after the first year in soil as the residual nitrogen not taken up by the crop. Accumulated res idual nitrogen is converted to nitrate-N and leached rapidly from the soil profile during the wet season following the harvest. To reduce leaching of the residual nitrate, the rates, frequencies and timings of fertilizer appl ication and irrigation must be scheduled in accordance with the plant growt h periods and the hydraulic regime of the soil.